A bit confusing, this one.
4B0EF4 does show as HB-FRG on Airframes, but to the right of the panel it shows as now re-registered N-730AG. Click on N-730AG and up comes Mode S code A9CB121 which of course isn't showing on RB screen.
Perhaps reverted to HB- FRG?
No, the explanation is much simpler.
HB-FRG is a frequently re-used Pilatus test/delivery registration. It's currently worn by PC-12 MSN 1241. Airframes doesn't list this one, but it does shown three previous uses of this registration, on MSNs 718, 1015 and 1133, which became N324BK, N888SK and N730AG respectively.
Since the US is one of Pilatus's biggest markets, it's more than likely that MSN 1241 will end up with an N registration too in due course.